1. Field of the Invention
The present invention relates to a method for detecting a string with an underperforming PV module within a larger PV system with a plurality of electrically parallel-connected PV units (S, F), and with a total current measuring device for measuring the total current through all PV units of the parallel circuit and to an associated device for carrying out the method.
2. Description of the Background Art
Large PV systems may comprise thousands of PV modules, which must be measured individually, to identify and localize a faulty module. This effort is necessary, because the presence of a single module or also a plurality of underperforming, if not even defective, modules is no longer noticed in the total output. A defective module, in which a photovoltaic cell is nonconducting or in which the soldered connection between two cells is broken, leads to failure of the entire string of, e.g., 10 PV modules connected in series, because a single interruption also interrupts the series connection. At a total output of 2 MW of the photovoltaic system, the contribution of a string of, e.g., 2 KW corresponds to a thousandth part of the output. Even a plurality of defective or underperforming strings over time are not noticed immediately, because the deviation in the generated output can also be caused by weather and the measuring accuracy of the employed transformer when measuring currents in the lower range of the measuring range, inherent to the measuring device, is not sufficiently accurate to detect such small deviations even in unchanging weather. The deviations in the typically employed current measuring transformers, based on magnetic fields, are due to design. Permanently installed systems for power measurement are associated with an unjustifiable expense.
Apart from the above problem of the undetected low performance of the PV system, it is important, particularly during the warranty period, that justified claim options are recognized to make the manufacturer responsible for the defective PV module.
Various options for testing the performance of PV modules are known from the conventional art. In all methods that produce a meaningful result, the PV system must be decoupled from an inverter and connected to a measuring device.
Used as a measuring device in this case are multimeters, which measure the short-circuit current and the open circuit voltage of a PV module, a string with a plurality of PV modules connected serially behind one another, or a PV unit. The purpose of this measurement is to detect the basic function of the PV module, string, or the PV unit.
If the performance of a PV module, string, or a PV unit is to be determined, this occurs preferably by the connection of a U-I characteristic measuring device, which can measure the corresponding U-I characteristic. The thus measured characteristic is supplemented by the measured value of an irradiance sensor or a reference solar cell and the measured value of a temperature sensor, which measures the temperature of the PV module. The STC output value (standardized output value for photovoltaic modules) is calculated from the values mentioned above for irradiance, temperature, voltage, and current. This has a high inaccuracy, however, because of the many tolerances in the sensors used for the calculation.
Further, measurement with a current probe, with which the current strength of a PV module, string, or a PV unit can be determined during operation, is known from the state of the art. However, because voltage, irradiance, and temperature are not known, this type of measurement is sufficient only for the mere testing of functionality and the testing of fuses. All of the methods and devices in the prior art are not a sufficiently satisfactory solution in regard to their accuracy, the time needed for the measurement, and their applicability during operation.